Gas lifting is an upstream production activity which involves the pumping of gas through a pipework annulus to inject it into a mandrel on a riser between a wellhead and processing equipment. The gas is of a lower density than the medium into which it is injected and thus effectively lowers the density of the material in the riser. This injection therefore lowers the pressure required to “lift” the resulting material blend to the surface and promotes increased production, by up to 50% in some cases. Because the gas injected returns to the process with the additional production, it is effectively a recycle stream. Therefore, increasing the gas lift by 1,000 standard cubic feet of additional gas will result in 1,000+x standard cubic feet returning through the process.
This means that, although increasing the gas liftrate increases the production, it also increases the loading on the compression system. There is a limitation on the benefits of gas lifting a well. If the gas lift rate is increased too far, then the production will drop because the gas rate is actually throttling the production riser since the physical volume of material flowing through the pipeline creates a high pressure drop.
When there are multiple risers being gas lifted, the determination of the optimal amount of gas lift per well is extremely difficult. The dynamic constraints of the ambient temperature, gas density and back pressure on the pipeline all affect the capacity of the compression system. Coupling the dynamic capacity of the compression process with the determination of the optimal gas lift rate for each well and implementing the closest feasible optimum has not been possible previously. Moreover, over or under injecting gas into the wells can cause a reduction in the production rate of hydrocarbons, losing opportunity and decreasing the overall economic viability of the production site.